(Issued 1 Mar. 1980)
C 71
CRD-C 71-80
STANDARD TEST METHOD FOR
OF CONCRETE
1. Scope
This method covers the determination of
the ultimate tensile strain capacity of concrete
subjected to static and sustained flexural loading.
It is especially applicable to mass concrete and the
specimen dimensions are such as to facilitate
testing mass concrete.
Maximum variation from the nominal cross section
shall not exceed 1/8 inch (3.2 mm). Provisions
shall be made for positioning strain meters in the
molds parallel to the top and bottom surfaces (as
tested), 1-1/2 inches (38 mm) from those concrete
surfaces, centered within the middle third of a
60-inch (1524-mm) simple span (Figure 1), prior
to concrete placement.
2. Applicable Documents.
2.1 Handbook for Concrete and Cement.
CRD-C 4, Sampling Fresh Concrete.
CRD-C 20, Making and Curing Concrete
Test Specimens in the Laboratory.
CRD-C 16, Flexural Strength of Concrete
(Using Simple Beam with Third-Point Loading).
3. Significance and Use.
3.1 Cracking resulting from slow rates of
stress development is of primary concern to the
performance of mass concrete structures. The
current practice of the U. S. Army Corps of Engineers for determining the tensile strain capacity of
mass concrete is to perform rapid and slow flexure
tests on concrete beams. Results of these tests are
compared with tensile strains predicted by
analytical methods for a given structure. These
comparisons are the basis for development of
temperature and construction control procedures
for mass concrete projects.
3.2. When the purpose of the test is to
compare the strain capacity potential of different
concretes, loading should be started at 7 days age.
When the complete strain capacity behavior of a
given concrete is desired, start loading at 7 and 28
days age. When design conditions indicate the
necessity of desirability, starting loading at 90
days and later ages should be evaluated.
4. Apparatus.
4.1 Molds. The rectangular steel molds shall
be 12 by 12 inches (305 by 305 mm) in cross
section. The length shall be at least 4 inches (102
mm) greater than five times the depth as tested.
The inside surfaces of the molds shall be smooth
and free from blemishes. The sides, bottom,
and ends shall be at right angles to each other and
shall be straight and true and free of warpage.
Fig. 1. Position of strain meters in
beam mold
4.2 Loading Frame. The loading frame shall
be capable of applying and maintaining the
required third-point flexural load on the specimen,
(Issued 1 Mar. 1980)
2
TEST FOR ULTIMATE TENSILE STRAIN CAPACITY OF CONCRETE (C 71-80)
despite any change in the dimensions of the specimen. The apparatus should be capable of maintaining the specified span length and distances
between loading and support rollers constant
within ±0.1 inch (±2.5 mm). Reactions should
be parallel to the direction of the applied forces.
The testing machine may be of any type of sufficient capacity, and shall be capable of
providing the rate of loading prescribed in 6.1.
Sustained loads may be maintained using an oil
reservoir under regulated high-pressure gas,
springs, or deadweights. Hydraulic pressure gages
or load cells shall be provided for measuring loads
to the nearest 2 percent of the total applied load.
4.3 Strain Gages. Suitable surface strain gages or embedded strain meters shall be provided
for the measurement of longitudinal compressive
and tensile strain in the specimen to the nearest
five millionths. Surface gages can effectively be
used for rapid loading tests, but embedded strain
meters should be used for sustained tests to avoid
errors due to creep of surface-mounted gages. The
effective length of embedded meters or series
installed surface gages shall be at least three times
the nominal maximum size of aggregate in the
concrete. Strain meters for sustained load specimens shall be capable of measuring strains for at
least six months without change in calibration.
Note-Carlson Model M-10 Strain Meters (10-inch gage)
length have performed satisfactorily (Carlson Instruments, 1190-C
Dell Avenue, Campbell, CA 95008).
5. Test Specimens.
5.1 Number of Specimens. A minimum of
three beams shall be made.
5.2 Fabrication. Concrete mixing, placing,
consolidation, and finishing shall be in accordance
with applicable provisions of CRD-C 10. Mass
concrete with maximum size aggregate in excess
of 3 inches (76 mm) shall be wet-sieved through a
3-inch (76-mm) sieve in accordance with CRD-C
4, or laboratory mixtures reproportioned for 3
inches (76 mm) when maximum size aggregate is
in excess of 3 inches (76 mm).
5.3 Curing and Storage. After finishing,
specimens shall be moist cured in their molds at
73.4±3.0°F (23.0±1.7°C) for a period appropriate
for satisfactory handling based on mix and age at
loading. Beams will then be stripped, rotated 90
degrees, and sealed in a membrane to prevent
drying during storage and testing. Beams shall be
stored in the laboratory at a uniform temperature
preferably in a range of 68 to 77°F (20 to 25°C)
uniformly supported in a no-load condition until
the time of test.
5.4 Surface Mounted Strain Gages. Three
6-inch (152.4-mm) gage length strain gages are
mounted on the tension face and two on the
compression face (Note 1). Gages are mounted in
series centered in the middle third of the beam
span. Gages are bonded to the beam surfaces with
a rapid setting adhesive (Note 2). Gage mounting
and testing must be accomplished at least one-half
hour before moisture from the concrete returns to
the dried area and delaminates and/or grounds
out the strain gage. A satisfactory procedure is as
follows:
a. On the day prior to test, the beam is
removed from curing storage and enclosed in a
plastic wrap (Note 3).
b. Openings, approximately 1 x 20 inches (25
x 508 mm) on the tension face and 1 x 14 inches (25
x 356 mm) on the compression face, are cut in the
plastic wrap at locations for the strain gages.
c. The exposed concrete areas are thoroughly
washed with alcohol to remove form oil.
d. On the day of the test, the exposed areas of
the beam are thoroughly dried by spraying with
alcohol and/or ether and blowing with compressed air.
e. Gages are bonded by applying the adhesive
to the back of the gages, positioning the gages on
the beam faces, covering the gages with a piece of
wax paper, and applying pressure by means of
wooden blocks spanning the installed strain gages
and held in place by cross arm braces. The gage
bonding operation and beam test must be completed within one-half hour to preclude return of
moisture to the dried area causing delamination
and/or grounding out of the paper backed strain
gages.
Note 1-SR-4 Type A-9 Strain Gages, available from BLH
Electronics, Inc., Waltham, MA 02154, have performed
satisfactorily.
Note 2-Eastman Chemical Co. manufactures a cyanoacrylate
adhesive called Eastman 910, which will set satisfactorily in five
minutes.
Note 3-Bituthene, supplied by W. R. Grace Co., Cambridge,
MA 02140, has been satisfactorily used for sealing sustained load
beams. Saran plastic wrap has been used for sealing rapid load
beams.
6. Procedure.
6.1 Loading of the beams shall be accomplished in accordance with applicable provisions
of CRD-C 16. For each loading age, one beam
shall be loaded to failure in third-point flexure
using a rapid loading rate of 40 psi/min (0.28
(Issued 1 Mar. 1980)
TEST FOR ULTIMATE TENSILE STRAIN CAPACITY OF CONCRETE (C 71-80)
MPa/min.) A continuous record of load and
strain may be obtained throughout the test using
an X-Y plotter, magnetic tape, etc., or strains may
be recorded manually after each 500 lbf (2000-newton)
increment of total load. A second beam shall be
started in its slow-loading cycle (Figure 2) at the
same age and the third beam stored under no load.
In the slow-loading cycle, 25-psi (0.17-MPa) fiber
stress shall be applied to the beam weekly until
failure occurs. Take strain readings immediately
before and after application of load increments.
Intermediate readings may be taken if necessary.
Measure the load prior to making each reading. If
the load varies more than 2 percent from the
correct value, it must be adjusted. Take weekly
strain readings on stored specimens and plot
strain versus time. At the time of failure in the
second beam, test the third beam to failure using
the 40-psi/min (0.28 MPa/min) loading range.
After failure, measure the test specimen in accordance with CRD-C 16.
7. Calculations.
7.1 The modulus of rupture shall be calculated
in accordance with CRD-C 16. If embedded strain
meters are used, outside fiber strains are extrapolated from the strain readings assuming a linear
strain distribution. Sustained load strain meter
readings shall be corrected for autogenous volume
C 71
3
change using the strain versus time plots for the
stored specimens. Plot tensile stress versus tensile
strain from data obtained from the test and take
the tensile strain at 90 percent of the ultimate load
as the ultimate tensile strain capacity.
8. Report.
8.1 The report shall include the following:
8.1.1 Identification number of each specimen.
8.1.2 Cement content, water-cement ratio,
maximum aggregate size, slump, and air content
of each batch.
8.1.3 Type and source of cement, aggregate,
and admixture for each mixture.
8.1.4 Storage conditions prior to and subsequent to loading.
8.1.5 Age at time of loading and, in the slowloading case, age at time of failure.
8.1.6 Type of strain gages and strain meters.
8.1.7 Average depth to the nearest 0.05 inch
(1.3 mm) for each specimen.
8.1.8 Average width to the nearest 0.05 inch
(1.3 mm) for each specimen.
8.1.9 Magnitude of total applied load and
modulus of rupture for each specimen.
8.1.10 Plot tensile stress versus tensile strain
data for each specimen.
8.1.11 Ultimate strain capacity for each test of
three beams.
Fig. 2. Slow-loading test apparatus